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United States Patent |
5,712,315
|
Dolan
|
January 27, 1998
|
Polytetrafluoroethylene molding resin and processes
Abstract
In this invention a new form of polytetrafluoroethylene has been found to
be compression moldable to provide strong molded articles. It is also
useful as an additive or strength binder. The new form is a compression
molding powder comprised of comminuted, sheared particles of expanded
porous polytetrafluoroethylene, said particles having a
nudular-microfibrillar structure of irregular shape, and a mean particle
size between 5 and 500 micrometers, and having a coating of a
thermoplastic fluoropolymer on at least a portion of the particle. By
irregular shape is meant that the nodes and fibrils do not have an ordered
arrangement. The bulk density is about 0.06 to 0.2 g/cc.
Inventors:
|
Dolan; John W. (Boothwyn, PA)
|
Assignee:
|
W. L. Gore & Associates, Inc. (Newark, DE)
|
Appl. No.:
|
546391 |
Filed:
|
October 18, 1995 |
Current U.S. Class: |
521/57; 521/59; 525/189; 525/199 |
Intern'l Class: |
C08J 009/22; C08J 009/16 |
Field of Search: |
521/57,59
525/189,199
|
References Cited
U.S. Patent Documents
3953412 | Apr., 1976 | Saito et al.
| |
3953566 | Apr., 1976 | Gore.
| |
3981853 | Sep., 1976 | Manwiller.
| |
4379858 | Apr., 1983 | Suzuki.
| |
4454249 | Jun., 1984 | Suzuki et al.
| |
4714748 | Dec., 1987 | Hoashi et al.
| |
4770922 | Sep., 1988 | Hatakeyama et al.
| |
4882113 | Nov., 1989 | Tu et al.
| |
5071609 | Dec., 1991 | Tu et al.
| |
5110527 | May., 1992 | Harada et al.
| |
5118788 | Jun., 1992 | Hosokawa et al.
| |
5156343 | Oct., 1992 | Sueyoshi et al.
| |
5242765 | Sep., 1993 | Naimer et al.
| |
Foreign Patent Documents |
0106180 | Sep., 1983 | EP.
| |
1082859 | Oct., 1964 | GB.
| |
Primary Examiner: Lee; Helen
Attorney, Agent or Firm: Samuels, Esquire; Gary A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of application Ser. No. 08/323,462 filed
Oct. 12, 1994 which application is now: pending, which is a
continuation-in-part of application Ser. No. 08/021, 409, filed Feb. 23,
1993, now abandoned.
Claims
I claim:
1. Process for preparing a molding resin which consists of providing a
porous, stretched polytetrafluoroethylene material having a coating of a
thermoplastic fluoropolymer thereon,
grinding and shearing said material into irregularly shaped, elongated
particulate in which the elongated particulate is entangled with one
another, and in which the elongated particulate has a mean particulate
size between 5 and 500 micrometers.
Description
FIELD OF THE INVENTION
This invention relates to polytetrafluoroethylene molding powders useful in
compression molding techniques.
BACKGROUND OF THE INVENTION
Polytetrafluoroethylene (PTFE) is made in two distinct forms by different
processes. One form is the so-called "fine powder" form produced by
polymerizing tetrafluoroethylene in an emulsion so that the polymer
particles do not precipitate. After polymerization is complete the
particles are coagulated. This form of PTFE cannot be compression molded.
On the other hand, the so-called granular form of polytetrafluoroethylene
is produced by polymerization of tetrafluoroethylene and precipitation in
situ as the polymerizate forms. This form of polytetrafluoroethylene can
be compression molded by taking the polymer powder, filling a mold,
compressing the powder in the mold while heating it to coalesce the
powder, and then removing the compressed powder from the mold. The
resulting molded articles are not as strong as desired and ways of making
stronger molded articles of polytetrafluoroethylene are continually sought
after.
SUMMARY OF THE INVENTION
In this invention a new form of polytetrafluoroethylene has been found to
be compression moldable to provide strong molded articles. It is also
useful as an additive or strength binder.
The new form is a compression molding powder comprised of comminuted,
sheared particles of expanded porous polytetrafluoroethylene, said
particles having a nudular-microfibrillar structure of irregular shape,
and a mean particle size between 5 and 500 micrometers, and having a
coating of a thermoplastic fluoropolymer on at least a portion of the
particle. By irregular shape is meant that the nodes and fibrils do not
have an ordered arrangement. The bulk density is about 0.06 to 0.2 g/cc.
To make the particles, a polytetrafluoroethylene substrate, usually in the
form of a tape or film, is contacted with a layer, usually a film, of a
thermoplastic polymer. The composition is heated to a temperature above
the melting point of the thermoplastic polymer, and, the composition of
step is then stretched while maintaining the temperature above the melting
point of the thermoplastic polymer. Finally, it is cooled.
Depending on the degree of stretching, the thermoplastic film can form a
very thin, i.e., 9 micron or less thick, film on the surface of the
expanded porous PTFE which is continuous and nonporous.
The coated film or tape is then slit into a fiber width which is suitable
for towing easily (i.e. 13 mm to 51 mm wide). Additional expansion of the
fiber is helpful before an optional towing step described below, to
increase the fiber's tensile strength.
The towing process provides an efficient means for size reducing the slit
tape into fine discontinuous fibers. The fine discontinuous fibers are
then further size reduced by shredding the fibers into fine staple fibers
(about 6 mm (1/4 inch) in length). This fiber length is suitable for
further size reduction in a colloid mill. Larger fiber lengths should be
avoided since they tend to classify within the colloid mill resulting in
decreasing the efficiency of the colloid mill. A colloid mill useful to
reduce the fine discontinuous staple fiber is a modified Morehouse Super
800 series colloid mill. The Morehouse mill can be modified by securing
the mill stones as is taught in U.S. Pat. No. 4,841,623 to Rine.
This modified colloid mill provides a means to size reduce the coated
expanded PTFE material to particle sizes down to submicron levels but
typically the mill is used to reduce material to particles of an average
size of about 40 micrometers. Larger particle sizes are attainable as well
e.g. 200 micrometers by varying the coarseness (grit size) of the
Additionally, this mill can be adjusted so as not to melt the
thermoplastic coating on the expanded PTFE material during the comminution
process. The problem of melting the thermoplastic material during
comminution is a common problem with other comminution methods and
processes.
The operation of the present invention should become apparent from the
following description when considered in conjunction with the accompanying
drawings:
This material can be used as a molding powder with increased strength due
to its superior matrix tensile strength resulting from the expansion
process and due to the thermoplastic coating which act as a binder and
aids in the strengthening of the molding powder as well. Additionally, the
material can be used as an additive or strength binder to other materials
such as to carbon black substrates, or to other polymers. The high
strength of the comminuted expanded PTFE fibers act as tensors in the
substrate and the fluoropolymer coating acts as a binder. Typically, a
comminuted particle possessing a defined aspect ratio is desired for this
type of application over a spherical or elliptical particle. A pronounced
aspect ratio defining the particle's geometry aids in the strength
enhancement due to the entangling of particles with themselves and/or
other materials when added to composites. The aspect ratio of a particle
is a dimensionless number greater than or equal to one and is defined as
the particle's length divided by its width or diameter. Additionally, the
particle's length is greater than its width or diameter.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of tensile strength versus temperature of compression
molded coupons showing that the effect of melting the thermoplastic
coating on the expanded PTFE particles increases the tensile strength of
the compression molded article.
FIG. 2 is a photograph of particles of the present invention enlarged 200
times, showing the irregular shapes and their entangling
FIG. 3 is an SEM of large particles of the present invention enlarged 100
time showing their definitive aspect ratio and entangling
FIG. 4 is an SEM of a cross-section of an entanglement of particles of the
present invention enlarged 2500 times showing the porous surfaces.
DETAILED DESCRIPTION OF THE INVENTION
The expansion, i.e., or stretching of polytetrafluoroethylene is a
well-known procedure and is described in U.S. Pat. No. 3,953,566.
Preliminarily, the type of PTFE called fine-powder, which may contain a
filler material, is mixed with a sufficient amount of a hydrocarbon
extrusion aid, usually an odorless mineral spirit until a paste is formed.
The paste is compressed into a billet and subsequently extruded through a
die in a ram-type extruder to form a coherent PTFE shape which can be in
the form of a rod, filament, tube or, preferably a sheet or a film.
The coherent PTFE shape is optionally compressed and then dried by
volatilizing the hydrocarbon extrusion aid with heat. Volatilization of
the extrusion aid results in a coherent PTFE shape having a small degree
of porosity. The resulting porous PTFE material is now ready to be coated
with the thermoplastic polymer and the coated material expanded. However,
if a highly porous expanded PTFE product is desired, a pre-conditioning
stretching step can be carded out by stretching at 200.degree.
C.-300.degree. C. preferably for about 1.5 to 5 times the original length.
The coherent PTFE shape prior to stretching is combined with a
thermoplastic polymeric layer by contacting the thermoplastic polymeric
layer with a surface of the coherent PTFE shape. Ordinarily the PTFE is in
sheet or membrane form and the thermoplastic polymer is in sheet or film
form and the polymer sheet is placed on the PTFE sheet. The combination is
heated to a temperature between the melt point of the thermoplastic
polymeric layer and about 365.degree. C. The coherent PTFE shape is kept
under tension while being heated thereby maintaining its dimensions while
the thermoplastic polymeric layer is combined with the coherent PTFE
shape. The means for heating the coherent PTFE shape may be any means for
heating commonly known in the art including, but not limited to, a
convection heat source. The combination heat source may be a heated
surface such as a heated drum or die or roll or curved plate. As the
coherent PTFE shape is heated to a temperature above the melt point of the
thermoplastic polymeric layer, the thermoplastic polymeric layer in
contact with the coherent PTFE at least partially softens and adheres to
the surface of the coherent PTFE shape thereby forming a composite
precursor, i.e., a coated PTFE material ready to be expanded. When a
conductive heat source is used as the means for heating the coherent PTFE
shape, the surface of the coherent PTFE shape should be against the
conduction heat source so as to prevent sticking and melting of the
thermoplastic polymeric layer upon the conduction heat source.
The thermoplastic polymeric layer is of a thermoplastic polymer that has a
melt point of 342.degree. C. or less. Melting points of thermoplastic
polymers were determined by Differential Scanning Calorimetry Techniques.
The thermoplastic polymer is a polymer that will bond to the substrate and
may be polypropylene, polyamide, polyester, polyurethane, or polyethylene.
Preferably, the thermoplastic polymer is a thermoplastic fluoropolymer.
Representative thermoplastic fluoropolymers include fluorinated ethylene
propylene (FEP), copolymer, copolymer of tetrafluoroethylene and
perfluoro(propylvinyl ether) (PFA), homopolymers of
polychlorotrifluoroethylene (PCTFE) and its copolymers with
tetrafluoroethylene (TFE) or vinylidene fluoride (VF2),
ethylene-chlorotrifluoroethylene (ECTFE) copolymer,
ethylene-tetrafluoroethylene (ETFE) copolymer, polyvinylidene fluoride
(PVDF), and polyvinylfluoride (PVF). Thermoplastic fluoropolymers are
preferred as the thermoplastic polymer since thermoplastic fluoropolymers
are similar in nature to PTFE, having melt points near the lowest
crystalline melt point of PTFE, and therefore are relatively high
temperature thermoplastic polymers. Thermoplastic fluoropolymers are also
relatively inert in nature and therefore exhibit resistance to degradation
from many chemicals.
The coated material is expanded by stretching it according to the method
taught in U.S. Pat. No. 3,953,566 to Gore. The temperature range at which
expansion of the material is performed is between a temperature at or
above the melt point of the thermoplastic polymer layer and a temperature
at or below 342.degree. C.
The material may be stretched uniaxially, only in a longitudinal direction;
biaxially, in both longitudinal and transverse directions; or radially, in
both longitudinal and transverse directions simultaneously. It may be
stretched in one or more steps.
The coherent PTFE shape forms an expanded porous PTFE (ePTFE) article as it
is stretched. The ePTFE article is characterized by a series of nodes
interconnected by fibrils. As the coherent PTFE shape is expanded to form
the ePTFE article, the thermoplastic polymer layer adhered to the coherent
PTFE shape is carded along a surface of the coherent PTFE shape while in a
melted state thereby forming a thin thermoplastic polymer film on the
ePTFE article. The thin thermoplastic polymer film is less than 9 microns
thick, and preferably has a thickness of one haft, preferably one tenth,
of the thermoplastic polymer layer's original thickness. For example, a
thermoplastic polymer layer originally having a thickness of 1 mil (25.4
microns) could produce a thin thermoplastic polymer film having a
thickness as low as 0.1 mil (2.54 microns) or less after expansion of the
coherent PTFE shape into the ePTFE article.
The thermoplastic polymer layer is in contact with and is carded on a
surface of the coherent PTFE shape as the coherent PTFE shape is expanded
at a temperature at or above the thermoplastic polymeric film's melt
point.
The coated material may be heat set, if desired, to amorphously-lock the
expanded porous PTFE structure.
Any suitable apparatus for grinding or comminuting tough polymeric or
elastomeric materials may be used for producing the porous expanded PTFE
particles, such as the apparatus disclosed in U.S. Pat. No. 4,614,310 and
U.S. Pat. No. 4,841,623. This apparatus employs two parallel stones having
a hole in the middle of the stones, affixed on a common axis but secured
circumferentially and rotating at high speeds (3600 rpm) in opposite
directions of each other. Material to be ground is coarsely cut and mixed
with water to produce a wet slurry and then the slurry is placed in the
middle of the rotating stones. The material is moved or slung by
centrifugal force across the surface of the stones. A hydrodynamic layer
is created between the closely spaced rotating stones and the water
slurry. This hydrodynamic layer forces the particles on the stone's
surface outwards across the stone. Size reduction of the particle occurs
as the particle bumps into and along the stone's sharp edges located on
the stone's surface as the particle travels along its torturous path from
near the center of the stone to the outside of the stone.
It is important that the gap between the stones is kept sufficiently tight
so that a strong hydrodynamic layer is maintained between the stones,
forcing the particles outwards. If the gap is not sufficiently tight, the
particles will ride in the center of the hydrodynamic layer and not touch
the stone's surface resulting in no size-reduction of the particle.
The overall amount of size reduction is a function of the stone's grit size
and the time the particle is exposed to the stone. The space between the
stone comes into play by maintaining a hydrodynamic layer between the
stones. Once the gap between the stones is sufficiently narrow to allow
size-reduction to occur, then any further gap narrowing will not lead to a
decrease in particle size. However, the gap dimension is critical to the
overall particle size variance. The tighter the gap between the stones
after the hydrodynamic layer is created, the more consistent mean particle
size is produced. This is due to the probability of particles entering and
leaving the hydrodynamic layer during the grinding operation due to the
strength of the hydrodynamic layer and the kinetic energy of the particle
as it transverses its path between the stones. The tighter the gap between
the stones, the less likelihood the particle can enter into the center of
the hydrodynamic layer and not be size-reduced.
It is useful to secure the mill stones of a Morehouse Super 800 series
colloid mill circumferentially as taught in U.S. Pat. No. 4,841,623 to
Rine, as opposed to securing them in the common axial mounting
configuration. The circumferentially mounting of the stones permits the
stones to withstand higher rotational velocities. When the grinding stones
rotate at the increased rotational velocities in the colloid mill, it is
found that expanded PTFE can be sized reduced to a mean particle size of
40 micrometers and smaller without severe degradation to the nodal-fibril
structure inherent to the expanded PTFE material.
Common size reduction techniques for PTFE and plastics use radiation to
render the material brittle to allow the material to be size reduced to
particles sizes below 100 micro-meters. Unfortunately, the irradiation
process destroys the nodal--fibril structure of the expanded PTFE
material. Excessive heat during the irradiation process is not a desirable
condition as well since melting of the thermoplastic and or
fluorothermoplastic may occur resulting in particle agglomeration. The use
of the modified Morehouse Colloid mill provides an alternative to the use
of irradiation to yield particles of sized reduced PTFE and size reduced
expanded PTFE below 100 micro-meters.
The comminuted particles of porous expanded PTFE retain their
nudular-fibrillar microstructure. The particles are characteristically
irregularly shaped and may be somewhat ragged as a result of shearing and
splitting the porous expanded PTFE pieces during the grinding process. The
particles have a preferred aspect ratio of between 3 and 50 and are
entangled. The comminuted ePTFE material, with its high surface area and
fibrillar, porous nature, is suited for use as an adhesive due to the
adhesive nature of the thermoplastic polymer.
The inventive material can be compression molded into articles of a desired
shape or geometry. It was found that the strength of the compression
molded article was increased if the molded shape was exposed to a
temperature above the melting point of the thermoplastic coating on the
particles. For an example, tensile test samples were made in the following
manner.
Approximately 6.0 grams of comminuted ePTFE material was placed in a steel
compression die so to produce tensile coupons the size of 12.7 mm (0.500
inch) wide by 101.6 mm (4.00 inch) long and approximately 2.28 mm (0.090
inch) thick. The final thickness of the coupon was dependent on the
quantity of material actually placed in the die. The width and length of
the coupon were maintained consistent due to the dimensional stability of
the compression die. The material was subjected to a compressive load of
27.6 to 34.5 MPa (4000 to 5000 psi) for a period of five minutes. The
direction of the compressive load was normal to the 12.7 mm by 101.6 mm
plane of the coupon thus reducing the thickness of the coupon to a
thickness of approximately 2.28 mm after compression. The five minute
compression load duration was used to permit any entrapped air within the
coupons to escape.
Coupons of comminuted ePTFE material having a coating of FEP were made in a
similar fashion.
Coupons were then heated at various temperatures 270.degree. C. To
350.degree. C. by placing the coupons in a forced-air electric oven Model
#7780 by the Blue-M Company of Blue Island, Ill. for a period of 30
minutes. The coupons were then removed from the oven and permitted to cool
down to ambient temperature under a laboratory hood having a hood face
velocity of 100 meter/min.
The tensile strength of the coupons were measured using a tensile tester
Model #1130 from the INSTRON Corporation of Canton, Mass. The INSTRON
machine was outfitted with damping jaws which are suitable for securing
the coupons during the measurement of tensile loading. The cross-head
speed of the tensile tester was 25.4 cm per minute. The gauge length was
44.5 mm.
As shown in FIG. 1, as the coupon is exposed to a temperature above the
melt temperature of the FEP, the strength of the molded article is
increased whereas the strength of the molded article consisting of
uncoated particles remains consistent until the article reaches
temperature at and above the coalescing temperature (327.degree. C.) of
the PTFE. This improvement in tensile strength is shown discretely for the
coupons heated to 300.degree. C. The FEP coating of the small particles
act as a binder. This improvement in tensile strength is advantageous and
provides an alterative for articles which can not be processed at
temperatures where PTFE is needed for coalescing to occur but can
withstand temperatures where melting of the thermoplastic coating can
occur.
For use in vacuum compression molding polytetrafluoroethylene, the new
comminuted porous expanded PTFE coated particles are placed in a mold of a
desired shape. A vacuum can be drawn, if desired, and then the material
compressed at pressures of between 1500 and 1600 psi (100 and 412 bar) at
a temperature between 20.degree. (ambient) and 380.degree. C. and for a
time of between 1 second and several minutes to reach equilibrium. With
the use of vacuum and heat, lesser compression loads are required to reach
densities greater than 1 g/cc to full density 2.2 g/cc.
Upon removal from the hot mold, the molded article can be cooled then
sintered, or can be directly sintered without cooling.
By sintering is meant that the molded article is heated above 327.degree.
C. for a period of time to reach equilibrium thermally to coalesce the
ePTFE particles.
In one type of molding operation, called hot isostatic molding, the
comminuted, porous stretched PTFE particles are placed in a container and
enclosed in an air-tight heat resistant wrapping. A vacuum is then drawn
on the enclosed material to about 20 inches of mercury (670 millibar). The
enclosed vacuumed material is then pressurized in an autoclave to about
200-275 psi (14-19bar) for 30-60 minutes at 350.degree.-400.degree. C. The
molded part is then removed and cooled.
In another type of molding operation, called compression molding, the
comminuted, porous stretched PTFE particles are placed in a mold and
compressed to 1450-2500 psi (100-172 bar) at room temperature
(15.degree.-25.degree. C.). If desired, the compression can take place
when a vacuum is pulled on the article. If desired the compression molded
article can be sintered at above the 327.degree. C. or at the or above the
melt temperature of the thermoplastic coating.
The thermoplastic fluoropolymer is preferably a copolymer of
tetrafluoroethylene and hexafluoropropylene (FEP) or a copolymer of
tetrafluoroethylene and perfluoro alkyl vinyl ether.
EXAMPLE
A fine powder PTFE resin was combined with a quantity of an odorless
mineral spirit and mixed until a paste was formed. The paste was
compressed under a vacuum to form a billet, and the billet was
subsequently extruded through a die thereby forming a coherent PTFE
extrudate.
The coherent PTFE extrudate was compressed between a pair of rollers until
a coherent PTFE sheet, 430 micron (0.43 mm) thick, was obtained. The
coherent PTFE sheet still contained an amount of the odorless mineral
spirit.
The odorless mineral spirit was volatilized from the coherent PTFE sheet by
heating, yielding a dry porous coherent PTFE sheet. Subsequently, the dry
coherent PTFE sheet was stretched while still hot two (2) times its
original length by passing the sheet over a series of gapped rollers
driven at differing speeds.
A sheet of a thermoplastic polymer, a copolymer of tetrafluoroethylene and
hexafluoropropylene (FEP) 25.4 micron (0.0254 mm) thick (available from
E.I. du Pont de Nemours, Co.) was slit so that its width was slightly less
than the width of the dry coherent PTFE sheet. The FEP sheet was fed on
top of the dry coherent PTFE sheet which in turn was fed across a heated
curved plate, heated to a temperature of 340.degree. C. which is above the
melting point of the FEP sheet, The two sheets were stretched together 1.2
to 1 to form a laminate. The speed with which the sheets were passed over
the heated curved plate was 12.19 m/min. The laminated sheets were slit in
half into 100 mm wide strips and stretched in two sequential heating
zones; one set at 335.degree. C., the other set at 335.degree. C. as well.
The laminate was stretched thirty (30) times its initial length, nominally
20 to 1 on the first plate and 1.5 to 1 on the second plate. The laminate
was raised over a third plate at 400.degree. C. and 19 meter/minute.
The strips of tape were subjected to a towing procedure. They were run
across a rotating roller. The rotating roller (152 mm diameter by 305 mm
long) contained hundreds of pins (0.6 mm diameter by 13 mm exposed length)
extending perpendicular to the axis of the roller. The pins were lined up
in a series of rows over the surface of the roller (they can be placed in
a random fashion over the roller's surface as well). The preferred
rotation of the tow machine roller is the direction opposite the direction
of the tape moving over the roller.
The tape was punctured and a series of discontinuous slits occurred in the
tape as each pin punctures the moving tape and exits the tape as the
pin-roll rotates. This discontinuous puncturing operation provides a
useful means of slitting the tape by not creating loose ends of threads
from the tape. After the towing operation, the tape is rendered as a
spider web like structure,
The expanded tow material was then spooled using a normal fiber take-up
machine. The denier of the coated tow material was 11,000 grams/9000
meters (Denier), or equivalently, 1222 dTex where
1 dTex=1 gram/1000 meters,
The tow fiber was cut into 6.35 mm long staple using a mini staple cutter
which consist of a 152.4 mm by 12.7 mm diameter (6" by 1/2" diameter)
feeding tube which directs the tow fiber at a rotating 25.4 mm by 76.2 mm
long (1" by 3" long) cutting blade. The tow material is fed into the
fading tube by two rotating nip rollers set at a speed so to produce 6.35
mm long staple fiber after it is cut by the rotating cutting blade. The
faster the nip rollers rotate feeding the tow fiber into the rotating
cutter results in longer staple fiber.
The 6.35 mm long fine staple fibers were then further sized reduced using a
modified Morehouse Super 800 series colloid mill. The Morehouse mill is
modified by securing the mill stones as is taught in U.S. Pat. No.
4,841,623 to Rine, as described further above.
Tap water was added to the sized reduced material in the hopper which feeds
the colloid mill. A water and ePTFE slurry was produced in the hopper with
a concentration of water to ePTFE of 50:50. Note that the higher the
concentration of ePTFE to water is made, the better the efficiency of the
mill. Although there does exists a peak concentration ratio since too much
ePTFE to water ratio will result in excessive heat build-up on the stones.
The stone must be kept cool during the milling operation. Excessive heat
build-up in the stones will render the stone useless as well as the heat
will melt the thermoplastic coating on the ePTFE material. The preferred
water and PTFE concentration is 35-40% PTFE to 65-60% water to allow for
adequate cooling of the stones and the coated ePTFE material.
The size reduced material exits the mill as a slurry. This slurry material
was then placed on flat aluminum pans in a forced air convection oven at a
temperature of 105.degree. to 150.degree. C. (or a temperature below the
melting temperature of the thermoplastic or fluorothermoplastic material
coating on the ePTFE material thus preventing sticking and particle
agglomeration due to melting of the thermoplastic material) and remains
there until the water evaporates.
The pans were removed from the oven and the cake like material was
fractured by blending the material using a standard household food
blender. The material was sometimes fractured as well by shaking it in a
closed container.
The product was a comminuted porous expanded PTFE material comprising
finely ground particles of irregular shape.
The comminuted particles preferably will have a mean particle size between
5 and 500 .mu.m, more preferably between 80 and 150 .mu.m. Particle size
was determined as follows: using a magnetic stirrer and ultrasonic
agitation, 2.5 grams of milled ePTFE powder were dispersed in 60 ml
isopropyl alcohol. (Ultrasonic Probe Model W-385, manufactured by Heat
Systems-Ultrasonics, Inc.). Aliquots of 4-6 ml of the dispersed particles
were added to approximately 250 m of circulating isopropyl alcohol in a
Leeds & Northrup Microtrac FRA Particle Size Analyzer. Each analysis
consisted of three 30 second runs at a sample circulation rate of 2
liters/minute scattering by the dispersed particles is automatically
measured and the particle size distribution automatically calculated from
the measurements.
The particles will preferably have an average surface area of between 1 and
4 sq. m/gram as determined by specific surface area measured by the
Micromeritics surface area analyzer. The surface area analyzer uses the
BET(1) method to calculate surface area. In this sample analysis, the
desorption isotherm of a single point analysis was used to calculate the
surface area.
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